Milliner’s ankle is sprained, which seems like a more promising diagnosis than a break, except that the Jets don’t yet know what type of sprain it is.

Milliner’s ankle is still too swollen for him to have an MRI examination, according to coach Rex Ryan. Once the swelling subsides, the MRI will presumably determine if Milliner has a traditional lateral low ankle sprain, or a more serious high ankle sprain, which generally sidelines players for longer.

The diagnosis of the variety of ankle sprain could determine Milliner’s availability for the Jets’ Sept. 7 opener against the Oakland.

Moreover, from Weeks 2 through 7, the Jets face a string of successful quarterbacks – Aaron Rodgers, Jay Cutler, Matthew Stafford, Philip Rivers, Peyton Manning and Tom Brady – who will test their secondary, especially if Milliner is not in it

So what is the difference between a lateral low ankle sprain and the more serious high ankle sprain?

According to athletic trainers, a lateral sprain happens when you roll your ankle, stretching the ligaments on the side of it. A high sprain happens when the ankle and leg rotate in opposite directions, stretching or tearing the ligaments that hold together the tibia and fibula, the two bones in the leg.

Recovering from a high sprain is difficult because, unlike with a lateral sprain, any pressure on the ankle further aggravates the injured ligaments.

Trainers can use aggressive rehab techniques with a lateral sprain, rotating the ankle to increase range of motion. Not so with a high sprain. Even stepping causes the talus joint, just beneath the tibia and fibula, to push on the tibiofibular ligaments, causing soreness in the front or back of the ankle.

Ten to 20 percent of all ankle sprains are high sprains. Studies show the average recovery time to be a month. Treatments range from using a removable protective boot to placing the ankle in a cast. Holding the tibia and fibula together can be critical, because whenever the bones separate, new bone can calcify between them, increasing recovery time by up to two weeks.

Fourteen years ago, when Jets backup quarterback Michael Vick played at Virginia Tech, the team’s trainer, Mike Goforth, borrowed a trick from the Minnesota Vikings and devised a brace using the clasp from the top of a ski boot. It helped hold the two leg bones together during games – something the ligaments would normally do – but Goforth had to release the clasp every four plays because it cut off blood circulation.

For Milliner, Ryan said his return date is “really up in the air, because we don’t know if it’s a high ankle (sprain), a low ankle.” Milliner spent Monday’s practice wearing a walking boot on his left foot, and pedaling slowly on a stationary bike.

Milliner’s injury occurred Sunday when he jumped to defend a pass for rookie receiver Quincy Enunwa. When Milliner fell to the ground, he was not sure what had happened to his ankle – a break or a sprain.

“I know it just hurt and I was in a lot of pain at the time,” Milliner said. “The player came down on my leg. It was caught up under his body.”

Milliner said he last hurt his ankle as a freshman at Alabama, though it was not as severe as his current injury.

He is an important piece in the Jets’ defense, since he is now their No. 1 cornerback, after the Jets cut Antonio Cromartie in the offseason, and replaced him with journeyman Dimitri Patterson.

Milliner’s rookie season last year was hampered by injuries. He missed spring workouts because of shoulder surgery. Then he missed chunks of training camp because of a calf injury. During the season, he was sidelined for three games by a hamstring injury.

After Milliner was benched three times, his late-season progress – three interceptions in the final two games – left teammates encouraged about his potential for this year. In the Jets’ preseason opener against the Colts last Thursday, Milliner performed well. He broke up two passes, including one in the end zone that he probably should have intercepted, Ryan said.

Weak-side linebacker Demario Davis has taken to calling Milliner “Franchise,” Davis said. That’s how much better the Jets expected Milliner to be this season, as compared to his rookie year. The Jets drafted him ninth overall last year, so they clearly have high hopes for his role in their defense in the coming years.

Milliner said it was “most definitely” frustrating to be sidelined by another injury.

“I want to be out here with my teammates in training camp,” he said. “Unfortunately, things like this happen all the time. It’s just a minor setback. I felt great (before the injury). I was doing good for the most part, practicing good, did good in the first game. I was working toward getting better each day. For something like this (to happen), it’s frustrating to me, because I want to be out there. I want to be involved.”

Milliner is also cautious about returning too quickly from this injury.

“You’ve got to be very cautious,” he said. “You don’t want to get out there and rush things and mess it up even more, get out there and you’re not 100 percent and give up plays. So I’m just going to be cautious with it and do what the trainers tell me.”